The invention relates to high voltage semiconductor devices and the manufacturing process thereof and, in particular, to a lateral double-diffused metal oxide semiconductor (LDMOS) transistor, particularly to vertical discrete LDMOS with source on the device backside. The device is considered ‘vertical’ because the source is on the bottom and the drain is on the top (or vice-versa). The term ‘lateral’ refers to the planar gate of the device.
Lateral double-diffused metal-oxide-semiconductor (LDMOS) transistors are commonly used in high-voltage applications (e.g. 20 to 500 volts or higher) because of their high breakdown voltage characteristics and compatibility with complementary metal-oxide-semiconductor (CMOS) technology for low voltage devices. In general, an LDMOS transistor includes a planar polysilicon gate, an N+ source region formed in a P-type body region, and an N+ drain region. The N+ drain region is separated from the channel formed in the body region under the polysilicon gate by an N drain drift region. It is well known that by increasing the length of the N drift region, the breakdown voltage of the LDMOS transistor can be accordingly increased.
Typically LDMOS transistors are used in high frequency applications, such as radio frequency and/or microwave power amplifiers. They are often employed in power amplifiers for base-stations where the requirement is for high output power with a corresponding drain to source breakdown voltage usually above 60 volts. It is desired, therefore, to provide LDMOS transistors that are capable of providing high frequency operation while maintaining the high voltage operation of the same.
It is desired in some cases to make the LDMOS transistor a vertical device. It can be especially advantageous to route the source to the bottom of the die for better packaging options, e.g. for lowering inductance on the source. However it is difficult to route the source of an LDMOS transistor to the substrate without a large increase in resistance.
Accordingly, there is a need to provide improved LDMOS transistors.